This invention is generally in the field of physical-chemical treatment, and relates to a method of obtaining tantalum, niobium or tantalum-niobium powders with a highly developed surface. The invention is particularly useful for manufacturing electrolytic capacitors.
Capacitors serve for storing an electrical charge for a later use. Electrolytic capacitors are typically manufactured from aluminum or tantalum powder. Tantalum and niobium (as well as tantalum-niobium) capacitors have become the preferred type, due to their relatively high reliability, long service life, and high capacitance and voltage ranges at relatively small dimensions, as compared to the aluminum ones. More specifically, tantalum capacitors have as much as three times better capacitance/volume efficiency than the aluminum capacitors.
Generally, a capacitor is formed by two spaced-apart conducting surfaces (usually metal plates) serving as electrodes, and the space between them is filled with an insulating or dielectric material. The dielectric used in tantalum capacitors is tantalum pentoxide that possesses high dielectric strength and a high dielectric constant. As capacitors are being manufactured, a film of tantalum pentoxide is applied to the electrodes by means of an electrolytic process.
The capacitance, C, of a capacitor is determined by the surface area of the two electrodes, the distance therebetween and dielectric constant of the insulating material, that is   C  =            ϵ      ·      A        t  
wherein xcex5 is the dielectric constant, A is the surface area; and t is the distance between the electrodes.
In tantalum electrolytic capacitors, the distance between the electrodes is very small, since it is only the thickness of the tantalum pentoxide film. As the dielectric constant of the tantalum pentoxide is extremely high (xcex5≈26), the capacitance of the tantalum capacitor is also very high and can be increased even more, if the surface area of the electrodes is increased.
Tantalum capacitors contain either liquid electrolytes (sulfuric acid) or solid electrolytes forming the cathode electrode, while the anode is formed by the tantalum pellet provided with a lead wire embedded in or welded to the pellet. This anode has an enormous surface area for its size because of the way it is made. The entire construction is enclosed in a hermetically sealed case.
The above construction, utilizing for example a liquid electrolyte, is typically manufactured in the following manner. Tantalum powder of suitable fineness, typically 2-5 xcexcm, (sometimes mixed with a binding agent) is machine-pressed into pellets. The next step is a sintering operation in which binders, impurities and contaminants are vaporized, and the tantalum particles are sintered (welded) into a porous mass with a very large internal surface area. During this step, metallic links and electrical contacts between tantalum particles are created. In addition, a tantalum wire is introduced into the powder prior to sintering so as to form a contact of the future anode. In some cases, the lead is embedded during pressing of the pellet before sintering. A film of tantalum pentoxide is electrochemically formed on the surface area of the fused tantalum particles. Provided that sufficient time and current are available, the oxide will grow to a thickness determined by the applied voltage. The pellet is then inserted into a tantalum or silver can, which contains an electrolyte solution. Most liquid electrolytes are gelled to prevent the free movement of the solution inside the container and to keep the electrolyte in intimate contact with the capacitor cathode. A suitable end-seal arrangement prevents the loss of the electrolyte.
As to the solid electrolyte based tantalum capacitors, the electrolyte is usually a manganese dioxide formed on a tantalum pentoxide dielectric layer. This construction is manufactured by impregnating the pellet with a solution of manganous nitrate, and then heating the pellets in an oven, thereby converting the manganous nitrate into manganese dioxide. The pellet is next coated and sealed.
The tantalum capacitor can be either of a volume or of a foil type, in which case the anode is formed by a tantalum foil.
Various techniques for the manufacture of tantalum powders have been developed. They typically include the following stages:
dissolving the ore;
extraction and precipitation of tantalum hydroxide;
calcination of tantalum hydroxide and conversing it into tantalum pentoxide; and
reduction of the pentoxide into tantalum metal powder, e.g., with sodium vapor.
It is understood that the capacitance of a tantalum capacitor depends on the total surface area of a tantalum powder used in the manufacture of this capacitor. The larger the total surface area, the larger the capacitance. The total surface area of the tantalum powder can be increased either by decreasing the particles"" size, or by developing their surface area. However, the decrease of the particles"" size is limited to that (0.5.-1.0 xcexcm) required for providing electrical contact between the particles. When lowering the particle size, the possibility of contact breaking significantly increases. The reduction in particle size also suffers from the unavoidable requirement of high-energy treatment that is very complicated and expensive.
The surface developing techniques include the production of flakes or fragmentation-like powders with the particle size of 2÷50 xcexcm. Known techniques of surface development allow for achieving a roughness factor not exceeding 2. Maximal surface area of the tantalum powder that is obtainable by the conventional techniques is about 0.1-0.5 m2/g (BET).
Techniques aimed at increasing the surface area of a tantalum powder have been developed, and are disclosed, for example, in U.S. Pat. Nos. 5,261,942; 5,580,367 and 5,211,741. According to this technique, the BET surface area of a tantalum powder is developed up to 0.6 m2/g by producing a flaked tantalum and reducing the flake size by fracturing without substantially reducing the thickness or tapering the peripheral edges of the flakes. This is implemented by embrittling the conventional tantalum flake by hydriding, oxidizing, cooling to low temperatures or the like to enhance breakage when reducing flake particle size by mechanical means. Thereafter, the so treated tantalum powder is treated by mechanical means such as a vibratory ball mill, and then an additional temperature treatment is applied to the mechanically treated tantalum powder.
Further increase of the surface area of tantalum powder is desired. This, on the one hand, would allow miniaturization of electronic devices utilizing tantalum capacitors, and, on the other hand, would enable to obtain the same operational parameters of the capacitor whilst saving such an expensive raw material as tantalum.
There is accordingly a need in the art for a technique enabling to increase the surface area of a tantalum, niobium or tantalum-niobium powder.
The present invention takes advantage of the use of the principles of a mechanical alloying technique for developing the surface area of a selected metal powder obtained by any known technique.
The term xe2x80x9cselected metalxe2x80x9d used herein signifies a metal selected from a group consisting of tantalum, niobium, and mixtures thereof with the tantalum/niobium composition ratio (Ta:Nb) ranging between 1:9 and 9:1.
The main idea of the present invention is based on the following. The mechanical alloying technique generally consists of alloying powders of different metals without resorting to external heating or chemical processing. The present invention utilizes equipment commonly used in mechanical alloying processes, such as different mills, attritors, grinders, but uses a specific operational mode and specific additional powders to obtain a solid solution in which the selected metal particles are at least partly wetted with the additional powder. This additional powder is preferably a metal, such as Fe, Cu, Mn, Ti, Ni, Mg, Al, Zn, Cd, Co, Mo, having relatively weak corrosion resistance as compared to that of selected metal. In other words, the present invention utilizes such operational modes as to provide the alloying of the selected metal particles in their surface region only. The step of alloying is followed by a step of removing the additional powder (metal) from the surface of the selected metal particles. This may be implemented by chemical etching.
There is thus provided according to one aspect of the present invention, a method of treatment of an initial powder of a metal selected from a group consisting of tantalum, niobium and mixtures thereof for developing the surface of the initial selected metal powder particles, the method comprising the steps of:
(a) mechanically alloying the initial selected metal powder with a powder of an auxiliary substance having relatively weak corrosion resistance as compared to that of the selected metal, thereby providing interaction between the particles of the selected metal and the substance powders, wherein the mechanical alloying is continued until a solid solution of the selected metal and the substance is obtained, in which the selected metal particle has a substantially developed surface area and is at least partly wetted with said substance substantially within a surface region of the selected metal particle; and
(b) removing the substance from the obtained solid solution, thereby leaving the selected metal particles with the substantially developed surface area free from the substance.
Preferably, the substance is removed by chemical etching. The auxiliary substance may contain at least one metal or metal alloy. These at least one metal is either one of the following list: Fe, Cu, Mn, Ti, Ni, Mg, Al, Zn, Cd, Co, Mo or any suitable metal alloy, for example a steel powder or an amorphous powder.
Preferably, the chemical etching utilizes an acid solution as an active ingredient, for example HNO3, HCl or H2SO4, or a mixture of either two of them.
Preferably, the chemical etching comprises at least two sequential cycles, each next cycle starting with removing reaction products and adding an additional portion of the acid. The etching also includes heating of the reacting products up to a predetermined temperature, for example in the range 80-90xc2x0. Generally speaking, this temperature is such as to cause low boiling of the reacting products.
The method may also comprise the step of applying a decantation process to obtain a flushed powder of the selected metal, upon completing the etching, and may also comprise washing of the flushed metal powder by using a dialysis unit. This washing is preferably carried out with the applied voltages within the range 200-400V. The washing procedure proceeds until the specific resistance of water reaches the value of 10xe2x88x926 Ohmxe2x88x921cmxe2x88x921.
Additionally, the method may comprise filtering and drying of the obtained powder. The drying proceeds at the temperature of approximately 120xc2x0.
The method may also comprising the classification procedure aimed at removing particles having dimensions less than a desired value.
The method according to the invention enables to obtain the alloying depth of 10-20% of the selected metal particle""s size. The method may utilize initial selected metal powder having particles size in the range of about 0.2-50 xcexcm.
According to another aspect of the present invention there is provided a metal powder having substantially increased BET surface area prepared by the above-described method from an initial powder of a metal selected from a group consisting of tantalum, niobium and mixtures thereof, wherein the increased surface area reaches 40 m2/g and more.
According to yet another aspect of the present invention, there is provided an electrode having substantially increased surface area and capable of having substantially high specific charge, which electrode is fabricated from the above metal powder.
According to yet another aspect of the present invention, there is provided a capacitor containing the above electrode.